The invention relates to the detection and the estimation of the size of leaks in a heat exchanger between two fluids mainly in gaseous form, using a tracer gas which will generally be helium, while the exchanger is operating. A particularly important application of the invention is the detection of leaks in a heat exchanger for superheating the steam between two turbine bodies of a thermal power station.
Numerous processes are alreadly known for detecting leaks in heat exchangers. Many comprise checking by dilution of a tracer, which may be helium. This solution has drawbacks. It requires an accurate knowledge of the flow rates used and it is hindered by the recirculation of the fluids. Other known processes do not allow the size of the leaks to be determined, when they are small.
It is an object of the invention to provide a process and device for detecting leaks in an exchanger, allowing the appearance of a leak to be rapidly detected and its evolution followed, using only simple means and elements adaptable to several types of exchangers.
To this end, the invention provides a detection process in which a given amount of helium is injected in a short time into the high pressure fluid at the inlet of the exchanger and the possible presence of helium is determined in the low pressure fluid leaving the exchanger, once said injection has been carried out.
For assessing the size of the leak, a signal representative of the helium content of the low pressure fluid will be generally integrated in time after each injection. This operation may be carried out by a measurement device operating advantageously by mass spectrometry. For estimating or quantifying the leak, the detection device must be calibrated. This may be done by performing the same type of injection as for detection, but this time in the low pressure fluid upstream of the sampling point.
Because of the "pulsed" character of the injection, a costly device for premixing with known concentration may be dispensed with and possible accumulation and recirculation of the helium forming the tracer gas may be avoided.
In the application of the invention to the detection of leaks in a steam superheating exchanger, a fraction of the superheated steam is condensed at the outlet of the exchanger, gas is extracted from the condensation water and the presence of the tracer gas is sought in the uncondensable gses.
The invention also provides a device for detecting and assessing leaks in a heat exchanger when this latter is operating, comprising means for the temporary storage of a given amount of tracer gas, means for rapidly driving said amount into the high pressure fluid inlet of the exchanger, means for taking off a sample of the low pressure fluid at the outlet of the exchanger and for conditioning the fluid so as to extract therefrom the tracer gas, if any, and means for measuring the amount of tracer gas collected.
The measuring means comprise advantageously a mass spectrometer associated with a fixed flow sampling system or "breather" aspirating a part of the uncondensable gases after condensation of the fluid in the case where this latter is steam.
The whole of the device may be controlled by a programmable controller. This causes successive measuring sequences to be carried out, determines the involved amounts from data delivered by pressure sensors and detector means. The whole of the device may be carried on a vehicle for transport from one exchanger to another. In this case, it will be sufficient for the exchangers to have connections intended to receive flexible pipes belonging to the device.
The invention will be better understood from the following description of a particular embodiment, given by way of non limitative example.